Portable Electronic Resistance Meter
OBJECTIVE
A simple, easy to build portable all electronic resistance meter that can be used to accurately measure resistances up to a maximum of 1MΩ is the project under construction here. The circuit operates from twin 9V batteries. The resistance meter has a linear scale and the zero resistance point appears on the extreme left end of the scale. Another significant feature of this meter is that it can be used to measure resistances without actually taking them out of the circuit.It has six different selectable ranges that permit resistance measurement in decades of (i) 1Ω to 10 Ω (ii) 10 Ω t0 100 Ω (iii) 100 Ω to 1K (iv) 1K to 10K (v) 10K to 100K and (vi) 100K to 1M
CIRCUIT DESCRIPTION
The measurement of resistance in this meter is in terms of voltage developed across the unknown resistance due to a constant current flowing through it. The voltage is directly proportional to the unknown resistance value if the current is
constant. Different resistance ranges have been achieved by changing the amplitude of constant current i.e. the current remains constant only for a given resistance range. Also, the constant current amplitude is so chosen that in a given resistance range, it produces a voltage of 0.5Vfor the highest value of resistance in that range. That is, if you have selected a range of say 1K to 10K, then the amplitude of constant current for this range Will be 50 µA. This enables the user to use the meter without taking the components out of the circuit as 0.5V would not operate any of the silicon semiconductor devices (Diodes, Transistors etc.)
The constant current source is constituted by transistor (Q1), diode: (D1) and (D2), Zener diode (VZ1), resistors (R1) to (R7). Resister (R1) provides bias current for the zoner diode (VZ1) and diode (Dl). The amplitude of constant current is given by VZ1 divided by the resistance appearing in the emitter lead of (Q1) as the drop across (D1) cancels the (VBE) drop of (Q1).VZ1is a 5V zener diode and the resistors (R2) to (R7) are so chosen that the amplitudes of constant currents in six different range settings of switch (S1) are 50mA, 5mA, 500 µ A, 50 µ A, 5 µ A and 500nA. Six different ranges are (i) 0 to 10 ohms (ii) 10 ohms to 100 ohms (iii) 100 ohms to 1000 ohms (iv) lKto 10K(v) 10Kto 100K and (vi) 100K to IM. That is, 10Ω, 100Ω, 1000Ω, 10K, 100K and 1M resistors produce 0.5V across them and give full scale deflection in six different range settings. Resistors (R2) to (R7) should be precision resistors (±1% or better) as the accuracy of the instrument largely depends upon precision of these resistors.IC-1 if an opamp and is wired as a non-inverting amplifier with a gain of (1+R9/R8). The gain has been chosen to be (11) to produce 5.5 V at the opamp output for a full scale deflection in the meter. (C1) (C2), C3 and C4 are power supply decoupling capacitors. (P2) is the offset adjust potentiometer for the opamp. This potentiometer is used to calibrate the meter to read zero When a zero (or a short) resistance is connected between the unknown resistance terminals. (P1) is used to calibrate the full scale deflection end of the meter.
CALIBRATION
Connect a small piece of wire between the unknown resistance terminals to simulate a zero resistance. Select any of the resistance ranges.
Potentiometer (P2) is Adjusted to get a zero reading on the meter. Choose 10K full scale deflection range and connect a 10K, ±0.5% or ±1% resistor in place of shorted wire. Adjust (P1) to get full scale deflection on the meter. The meter is calibrated. If you have any difficulty in getting a precision 10K resistor for calibration, you could choose one from a general purpose resistor lot that is within ±0.5% of (OK.
CONSTRUCTION GUIDELINES
Figs. 16.2 and 16.3 respectively show the PCB layout the components layout.
PARTS LIST
Resistors and Capacitors
R1 : 330Ω, 1/4W
R2 : 100 Ω, 1/4W (Metal film)
R3 : 1 K, 1/4W (Metal film)
R4 : 10K, 1/4W (Metal film)
R5 : 100K, 1/4W (Metal film)
R6 : 1M, 1/4W (Metal film)
R7 : 10M, 1/4W (Metal film)
R8 : 1K, 1/4W
R9 : 10K, 1/4W
R10 : 10K , 1/4W
R11 : 1 K, 1/4W
P1, P2 : 10K, Multiturn trimmer potentiometer
C1 , C2 : 0.1µF (Ceramic disc)
C3, C4 : 10µF, 16V (Electrolytic)
Semiconductor Devices and its
D1, D2 : 1N4001 or equivalent
VZ1 : 5V, 400mW zener diode
IC-1 : opamp741 Q1 : 2N2907
Other components
Battery : Two 9V batteries
S1 : Rotary switch with one pole and at least six throws
S2 : DPDT switch
Meter : Meter with 1mA f.s.d.
Miscellaneous
8-pin D.I.L. IC base (1 No.), Multistrand wires, solder metal, leads with crocodile clips, suitable mounting cabinet.
Written by David on January 7th, 2010 with
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